A. Field of the Invention
The present invention relates generally to semiconductor devices, and, more particularly to a bipolar device having a shallow junction raised extrinsic base, and a method for making such a device.
B. Description of the Related Art
Bipolar transistors are three-region, two-junction devices. A typical heterojunction bipolar transistor (HBT) is fabricated by sequentially forming on a substrate, a collector layer or region, a base layer or region, and an emitter layer or region. An emitter electrode is formed on the emitter layer. A base electrode is formed on the base layer, on the same side of the substrate and adjacent to the emitter electrode.
Electrically, bipolar transistors can be thought of as two diodes back to back. The current flows from the emitter region through the base into the collector. When there is no current to the base, the transistor is turned off. When it is on, the current flows. It only takes a small current to turn the base on enough to allow current flow through the entire transistor.
Most bipolar circuits are designed with NPN transistors. NPN represents the respective conductivity types of the emitter, base, and collector. Bipolar transistors feature fast switching speeds. The speed and performance of a bipolar transistor is governed by a number of factors, including vertical base dopant film thickness (base width), the base resistance, and the base-collector capacitance.
One factor affecting the speed and performance of a bipolar transistor is the base width. Ideally, the base width is made as small as possible. Minimizing the base width is achieved through minimization of the deposited dimension (for a base layer formed through epitaxy deposition), minimization of the high temperature processing, and/or minimization of the quantity of silicon interstitial defects, which enhances diffusion of the base dopants.
Another factor affecting the speed and performance of a bipolar transistor is the base resistance. Reduction in the base resistance for a given emitter region size involves defining a self-aligned extrinsic base with low resistance, and forming the extrinsic base in close proximity to an intrinsic, higher resistance portion of the device. With close proximity of the extrinsic base to the intrinsic base, the base resistance may be reduced by up to fifty percent for narrow emitter bipolar devices. Further increasing the proximity of the extrinsic base to the intrinsic base could achieve even lower base resistance, except for the increasing influence the extrinsic base has on widening the intrinsic base through the inevitable presence of silicon interstitial defects in the extrinsic base. Thus, the reduction of base resistance through closer proximity of the extrinsic base to the intrinsic base eventually results in an undesirable tradeoff with wider base film thicknesses.
A third factor affecting the speed and performance of a bipolar transistor is the collector-base capacitance. The dominant element of this capacitance is the junction capacitance. The undesirable, or parasitic, element of this capacitance is the intersection of the extrinsic base with the collector region. This may be minimized with the shallowing of the extrinsic base diffusion.
Although improvements have been made individually in each of the factors affecting the speed and performance of bipolar transistors, simultaneous improvement of base width, base resistance, and base-collector capacitance has not been achieved. Thus there is a need in the art to provide a heterojunction bipolar transistor having simultaneously improved base width, base resistance, and base-collector capacitance over conventional bipolar devices.